The fabrication of various solid state devices requires the use of planar substrates, or semiconductor wafers, on which integrated circuits are fabricated. The final number, or yield, of functional integrated circuits on a wafer at the end of the IC fabrication process is of utmost importance to semiconductor manufacturers, and increasing the yield of circuits on the wafer is the main goal of semiconductor fabrication. After packaging, the circuits on the wafers are tested, wherein non-functional dies are marked using an inking process and the functional dies on the wafer are separated and sold. IC fabricators increase the yield of dies on a wafer by exploiting economies of scale. Over 1000 dies may be formed on a single wafer which measures from six to twelve inches in diameter.
Various processing steps are used to fabricate integrated circuits on a semiconductor wafer. These steps include deposition of a conducting layer on the silicon wafer substrate; formation of a photoresist or other mask such as titanium oxide or silicon oxide, in the form of the desired metal interconnection pattern, using standard lithographic or photolithographic techniques; subjecting the wafer substrate to a dry etching process to remove the conducting layer from the areas not covered by the mask, thereby etching the conducting layer in the form of the masked pattern on the substrate; removing or stripping the mask layer from the substrate typically using reactive plasma and chlorine gas, thereby exposing the top surface of the conductive interconnect layer; and cooling and drying the wafer substrate by applying water and nitrogen gas to the wafer substrate.
Photoresist materials are coated onto the surface of a wafer by dispensing a photoresist fluid typically on the center of the wafer as the wafer rotates at high speeds within a stationary bowl or coater cup. The coater cup catches excess fluids and particles ejected from the rotating wafer during application of the photoresist. The photoresist fluid dispensed onto the center of the wafer is spread outwardly toward the edges of the wafer by surface tension generated by the centrifugal force of the rotating wafer. This facilitates uniform application of the liquid photoresist on the entire surface of the wafer.
Spin coating of photoresist on wafers is carried out in an automated track system using wafer handling equipment which transport the wafers between the various photolithography operation stations, such as vapor prime resist spin coat, develop, baking and chilling stations. Robotic handling of the wafers minimizes particle generation and wafer damage. Automated wafer tracks enable various processing operations to be carried out simultaneously. Two types of automated track systems widely used in the industry are the TEL (Tokyo Electron Limited) track and the SVG (Silicon Valley Group) track.
The numerous processing steps outlined above are used to cumulatively apply multiple electrically conductive and insulative layers on the wafer and pattern the layers to form the circuits. The final yield of functional circuits on the wafer depends on proper application of each layer during the process steps. Proper application of those layers depends, in turn, on coating the material in a uniform spread over the surface of the wafer in an economical and efficient manner.
During the photolithography step of semiconductor production, light energy is applied through a reticle mask onto the photoresist material previously deposited on the wafer to define circuit patterns which will be etched in a subsequent processing step to define the circuits on the wafer. Because these circuit patterns on the photoresist represent a two-dimensional configuration of the circuit to be fabricated on the wafer, minimization of particle generation and uniform application of the photoresist material to the wafer are very important. By minimizing or eliminating particle generation during photoresist application, the resolution of the circuit patterns, as well as circuit pattern density, is increased.
A reticle is a transparent plate patterned with a circuit image to be formed in the photoresist coating on the wafer. A reticle contains the circuit pattern image for only a few of the die on a wafer, such as four die, for example, and thus, must be stepped and repeated across the entire surface of the wafer. In contrast, a photomask, or mask, includes the circuit pattern image for all of the die on a wafer and requires only one exposure to transfer the circuit pattern image for all of the dies to the wafer.
Reticles must remain meticulously clean for the creation of perfect images during its many exposures to pattern a circuit configuration on a substrate. The reticle may be easily damaged such as by dropping of the reticle, the formation of scratches on the reticle surface, electrostatic discharge (ESD), and particles. ESD can cause discharge of a small current through the chromium lines on the surface of the reticle, melting a circuit line and destroying the circuit pattern.
Reticles are transferred among various stations in a semiconductor fabrication facility in reticle pods, such as SMIF (standard mechanical interface) pods. SMIF pods are generally characterized by a pod door which mates with a pod shell to provide a sealed environment in which the reticles may be stored and transferred. In order to transfer reticles between a SMIF pod and a process tool in a fab, the pod is typically loaded either manually or automatically on a load port on the process tool. Once the pod is positioned on the load port, mechanisms in the port door unlatch the pod door from the pod shell such that the reticle may be transferred from within the pod into the process tool.
During transfer of a reticle between a pod and a process tool, it is desirable to minimize contact with the upper and lower surfaces of the reticle. Any such contact may generate particles and/or affect the pattern etched in the reticle. Any such contact may generate particles and/or affect the circuit pattern etched in the reticle. Accordingly, the engagement between the reticle and reticle forks for positioning the reticle must be minimal and precisely-controlled. It is therefore necessary to precisely position the reticle with respect to a reticle fork or other reticle gripping mechanism during transfer of the reticle.
FIGS. 1–3 show a reticle pod 12 which includes a pod shell 13 that contains a reticle 10. A removable pod door 14 seals the pod shell 13 and is supported on an indexer plate 20 of a process tool (not shown). The reticle 10 is secured to the pod door 14 typically by means of screws 18 that extend through screw openings (not shown) provided in the pod door 14 and are threaded through respective screw sleeves 16 and threaded into the reticle 10. The screw head 19 of each screw 18 is seated in a recess (not shown) in the pod door 14, and is normally flush with the bottom surface of the pod door 14. Prior to internalization of the reticle 10 into the mini-environment of the process tool, reticle-gripping members, such as reticle forks (not shown), engage and properly position the reticle 10 for internalization.
As shown in FIG. 1, the reticle 10 is normally disposed in a substantially horizontal plane prior to being gripped and positioned by the reticle-gripping members (not shown). This facilitates correct and precise contact between the reticle-gripping members and the reticle 10. However, as shown in FIG. 2, in some cases one of the screws 18 is incompletely threaded into the screw sleeve 16 that supports the reticle 10. This imparts a sloped or angled configuration to the reticle 10 such that improper contact between the reticle-gripping members and the reticle 10 occurs, causing potential damage to the reticle 10.
As shown in FIG. 3, another problem that sometimes occurs is that a particle or particles 22 inadvertently fall(s) on the indexing plate 20 prior to placement of the reticle pod 12 thereon. This may impart a tilting configuration to the pod door 14, and thus, the reticle 10, thereby causing improper contact between the reticle-gripping members and the reticle 10 and resulting in potential damage to the reticle 10. Accordingly, a reticle position detection system is needed for detecting or sensing the position of the reticle with respect to a horizontal plane prior to engagement of the reticle-gripping members with the reticle preparatory to positioning of the reticle for internalization into a processing tool.
An object of the present invention is to provide a reticle position detecting system which is capable of sensing or detecting a position of a reticle prior to internalization of the reticle into a process tool or other equipment.
Another object of the present invention is to provide a reticle position detecting system which is suitable for sensing or detecting a position of a reticle with respect to a horizontal plane to prevent damage to the reticle upon subsequent pre-positioning of the reticle for internalization into a process tool or other equipment.
Still another object of the present invention is to provide a reticle position detecting system which utilizes a laser beam to detect the relative position of a reticle in a reticle pod with respect to a plane.
Yet another object of the present invention is to provide a reticle position detecting system which includes a laser beam generator provided on one side of a reticle and a laser beam sensor provided on the opposite side of the reticle, wherein the laser beam generator emits a laser beam which is received or intercepted by the laser beam sensor in the event that the reticle pod is disposed in the correct position for engagement by a reticle fork or pre-alignment unit, and wherein the laser beam is reflected from the surface of the reticle and the laser beam sensor fails to receive or intercept the laser beam in the event that the reticle pod is disposed in the incorrect position for engagement by a reticle fork or pre-alignment unit.